FIG. 9A shows a representative example of a circuit configuration of a three-level inverter. In FIG. 9A, a capacitor series circuit is formed by two capacitors C1 and C2 as DC voltage sources being connected in series. A U-phase circuit 10U, a V-phase circuit 10V and a W-phase circuit 10W are connected to this capacitor series circuit in parallel.
The U-phase circuit 10U has switching elements U1 to U4 which are connected in series and diodes D1U and D2U which are connected in series between a common connecting point of the switching elements U1 and U2 and a common connecting point of the switching elements U3 and U4. A common connecting point of the diodes D1U and D2U is connected to a neutral point NP that is a common connecting point of the capacitors C1 and C2. A common connecting point of the switching elements U2 and U3 serves as a U-phase output end.
The V-phase circuit 10V has switching elements V1 to V4 which are connected in series and diodes D1V and D2V which are connected in series between a common connecting point of the switching elements V1 and V2 and a common connecting point of the switching elements V3 and V4. A common connecting point of the diodes D1V and D2V is connected to the neutral point NP. A common connecting point of the switching elements V2 and V3 serves as a V-phase output end.
The W-phase circuit 10W has switching elements W1 to W4 which are connected in series and diodes D1W and D2W which are connected in series between a common connecting point of the switching elements W1 and W2 and a common connecting point of the switching elements W3 and W4. A common connecting point of the diodes D1W and D2W is connected to the neutral point NP. A common connecting point of the switching elements W2 and W3 serves as a W-phase output end.
Each voltage value of the capacitors C1 and C2 is E. An end of the capacitor C1 at the switching element U1 side is +E level, the neutral point NP is 0 level, and an end of the capacitor C2 at the switching element U4 side is −E level. Each of the switching elements U1 to U4, V1 to V4 and W1 to W4 is formed by a self-arc-extinguishing type (or self-tern-off type) semiconductor device such as an IGBT.
In the circuit configuration shown in FIG. 9A, by an ON/OFF operation of each switching element, three level phase voltages of E, 0 and −E are outputted (with the neutral point NP being a reference of the phase voltage). An ON/OFF command signal of each switching element is determined by a comparison between a sine wave voltage command and two stages of carriers 1 and 2, as shown in FIG. 10. In FIG. 10, a thick solid line is a U-phase voltage command, a broken line is a U-phase current, and thin solid lines are the carriers 1 and 2.
Table 1 shows the ON/OFF command signals of the U-phase switching elements (U1 to U4) and the U-phase output voltages in comparison states between the U-phase voltage command and the carriers 1 and 2.
[Table 1]
The switching operation takes place when the switching element is turn ON (OFF→ON) or turn OFF (ON→OFF). That is, the switching operation takes place at a cross point between the voltage command and the carriers 1 and 2 shown in FIG. 10.
FIG. 10 shows the U-phase voltage command as a representative. Likewise, the ON/OFF signal commands of the switching elements for the V-phase and the W-phase are determined by a comparison between a V-phase voltage command or a W-phase voltage command and the carriers 1 and 2.
Here, a method of reducing a loss of the switching element in the inverter device has been proposed in, for instance, Patent Documents 1 and 2.